1. Field of the Invention
The present invention relates to an infrared sensor, and more particularly, to an infrared sensor in which a temperature can be compensated; and a method for compensating a temperature thereof.
2. Discussion of the Related Art
In general, measuring a temperature is closely related to our every day life, such as air conditioning and cooking as well as to industries. The temperature measuring is done by contact type or non-contact type sensors, of which used most frequently is the contact type and the non-contact type is used when a contact is not possible. For example, the non-contact type has been used for measuring objects to which a contact can not be easily made, such as rotating objects, moving objects, high temperature objects, and etc. As the non-contact type sensor is expensive and difficult to handle, the contact type sensor has been used widely than the non-contact type sensor. However, currently, the demand for the non-contact type sensors is increasing, particularly for simple and low cost radiation type sensors in comparatively low temperature measurements about 0.about.300.degree. C. Because these non-contact, radiation type sensors can be formed with a simple circuit and the infrared sensors applied thereto can be obtained at low cost currently, the radiation type sensors are more favorable than the contact type sensors in view of the cost. In the infrared sensor, there are photonic type sensors having the photovoltaic effect or the photoconductive effect applied thereto and thermal type sensors, such as bolometer, pyroelectric sensor and thermopile sensor. The photonic type sensor, which utilizes the electric characteristic change of the sensor caused by electron excitation due to incident radiation, has in general an excellent sensitivity and a fast responsivity within a selected wave band. However, the photonic type sensors have disadvantages in that the process technology has not been fully established, and the sensor costs are high and they should be operated at a temperature below liquid nitrogen for obtaining a desired infrared sensitivity. Therefore, in order to use an infrared sensor for commercial and industrial purposes, a sensor which is reliable and requires no cooling is required.
Currently, research for thermal type sensor which can satisfy such requirements are actively underway. As a result of such research, sensors have been developed which can provide useful information on objects that can not be perceived as a visible image, which can be utilized in fields, such as production examinations, process monitoring and non-contact and non-destructive testings. Of sensor material for these sensors, though the most excellent material is (Hg, Cd)Te, because the process technology has not be matured yet, provision of a homogenous substrate has been a problem. Accordingly, research for thermopile sensors that can solve the above problems while fabricated by a semiconductor process are underway. The thermopile sensor is a sensor for sensing a temperature utilizing the Seebeck effect in which two different materials with one ends in contact and the other ends opened generates a thermoelectric power in proportion to a temperature difference between the contact ends and the opened ends. The thermopile sensor has advantages in that it exhibits a stable responsive characteristic to a DC radiation, responds to a wide range of infrared spectrum, and requires no bias voltage or current. The operation principle of the thermopile sensor is based on the Stefan-Boltzmann's law that "every body radiates an energy proportional to a fourth power of an absolute temperature of the body". That is, P.varies..epsilon..sigma.T.sup.4, where T is an absolute temperature of the body, P is an energy radiated from the body, and .epsilon. is an emissivity. In conclusion, the thermopile sensor detects the energy proportional to T.sup.4 for measuring a temperature.
FIG. 1 illustrates energies incident to a background art thermopile sensor. As shown, in the energies incident to the thermopile sensor 1, there are an energy A radiated from a measured object 2, an energy B radiated from bodies around the measured object 2, reflected at the measured object 2 and incident to the thermopile sensor, an energy C radiated from a sensor package 3 at an ambient temperature and incident to the thermopile sensor 1, an energy D by thermal conduction through the sensor package 3, and an energy E radiated from the thermopile sensor 1 itself. Therefore, when an object at a high temperature is measured, the thermopile sensor provides a value proportional to a fourth power of the temperature T of the measured object according to the equation P.varies..epsilon..sigma.T.sup.4 presented above. However, when an object at a low temperature is measured, the thermopile sensor does not provide a value proportional to a fourth power of the temperature T of the measured object according to the equation P.varies..epsilon..sigma.T.sup.4 presented above, because there are, not only the energy A incident to the thermopile sensor from the measured object 2 under the Stefan-Boltzmann's law, but also the energy E radiated from the thermopile sensor 1 itself under the same law. This can be expressed as when P.varies..sigma.(.epsilon.T.sup.4 +RTs.sup.4 -To.sup.4), where To is a temperature of the thermopile sensor itself, Ts is an ambient temperature around the measured object, and R is a reflectance. That is, in a high temperature range where a temperature T of the measuring object 2 is very higher than a temperature To of the thermopile itself (T&gt;&gt;To), the To.sup.4 in the above equation can be neglected to measure the temperature of the measured object 2, in a low temperature range, the To.sup.4 can not be neglected and the temperature of the measured object 2 can not be measured accurately. Therefore, in order to compensate for components such as To.sup.4, the background art thermopile sensor is provided with a circuit for detecting and compensating for a sensor temperature or placed in a thermostatic chamber to maintain the sensor temperature constant. Currently, the circuitry compensation is widely used.
FIGS. 2 and 3 respectively illustrate block and circuit diagrams each showing the background art thermopile sensor with a temperature compensation.
Referring to FIGS. 2 and 3, the background art thermopile sensor with a temperature compensation is provided with a thermopile sensor 11 for sensing a temperature of a measured object, a sensor amplifying part 12 for amplifying a signal from the thermopile sensor 11, a temperature compensating element 13 for sensing an ambient temperature around the measured object, a temperature compensating part 14 for amplifying a signal from the temperature compensating element 13, a summing amplifier 15 for summing signals from the sensor amplifying part 12 and the temperature compensating part 14, and a constant voltage source 16 for applying power to the sensor amplifying part 12, the temperature compensating part 14 and the summing amplifier 15. The temperature compensating element 13 is formed with a thermistor or a diode. Thus, by compensating an ambient temperature around the measured objects, a temperature of a desired measured object could be measured.
However, the background art thermopile sensor with a temperature compensation has the following problems.
First, the circuit for a temperature compensation is complicated and costs high.
Second, the sensitivity should be intentionally lowered for preventing a temperature of an object around the measured object, including the temperature of the measured object, from being sensed when the thermopile sensor has a high sensitivity.